Rainy weather may be a boost for finding fossils

Our long, wet spring may have brought us a bumper crop of mosquitoes, but all that rain does have some benefit.

Besides ending last year's drought, those rains have been washing over Minnesota's fossil beds, clearing away the clay and dust, shifting pebbles and mud, bringing new fossils up to the light of day. It happens every year. As snowmelt and spring rains flow off the land, they leave a fresh crop of fossils unveiled, just waiting to be found by scrambling, energetic youth or older, patient eyes.

To make a discovery, though, it helps to look in the right places. Most of the rocks in our state are far too old, or (such as our granites) far too deep, to hold any trace of ancient life. Only to the southeast are the rocks different. There, east of New Ulm and from Minneapolis south, most of the rocks are sandstone, limestone and shale that formed only 450 million to 500 million years ago. And those rocks do have fossils, mostly in the limestone and shale layers.

As shale reverts to clay very quickly, the easiest fossils to find are those in the shale layers. The Decorah shale in particular is widespread and very rich in fossils. Patches occur high on the bluffs of Minneapolis and St. Paul, and it lurks farther south in a number of cliffs, claypits and roadcuts (check http://talc.geo.umn.edu/mgs/mnglance.html for possible collecting sites).

In the wild, this shale is not very impressive. Typically, it looks like a grayish-greenish mudbank if it's damp, or a dusty, hard-packed dirt if it's dry, and it is easily overgrown by grass and brush. If you look more closely, however, you'll find that most of the pebbles and gravel stuck in that dirt are fossils: mostly ancient sea shells, snail shells, twiggy bryozoans (a colony fossil somewhat like modern branching corals), or flat, washer-like crinoid rings (from the stems of ancient sea lilies). If you're lucky, you also might find a trilobite shell (something like a flattened, fossilized pill-bug), or the pointed, horn-like cone of an ancient coral.

The only problem with these fossils is they are fairly small.

While there are lots and lots of them, few are bigger than a dime.

That makes them easy to wash out of the rock, but it does rather limit the awe factor, no matter how exquisitely preserved they are.

Larger shells can be found in the limestones, but as those layers weather more slowly, they are harder to find and to liberate if the rock hasn't broken just right.

You also won't find any dinosaur bones. That's because the Decorah shale is over twice the age of the oldest dinosaur. As it's older than all but the simplest fishes, you're unlikely to find any bones there at all. For younger fossils you have to look elsewhere. Like maybe Hill Annex Mine State Park. There you can find shark teeth mixed in with Cretaceous seashells. So, keep your eyes open and good luck.

This is the opinion of Robert Wichman, a geologist and professor who has explored many of Minnesota's state parks. He can be reached via email at newsroom@stcloudtimes.com.